What value should I increase to reduce noise for subsurface scattering, volume scatter, etc.?
You can switch to branched path tracing and use its subsample controls. Increasing subsurface sample level will improve noise for SSS, volumes for volume scatter effects Note that volume samples only affect indirect light, meaning the noise that comes from volume bounces being > 0. For other volume noise, adjust lamp sampling values.

See this question for general info about branched path: What is branched path tracing and how is it useful?

I normally recommend using the "square samples" option with branched path, as it allows you to work with simpler values when juggling all the subsample counts. "square samples" just, well, squares all the values being used for sample counts. So "4" samples becomes 4*4=16 samples. The reason for this is the more samples you have, the more samples you need to add to get a noticeable difference. By adjust sample counts in increments of squares, you ensure you're always getting a statistically significant addition for each sample level.